The communication system is a cyclic, serial, multi-drop, master-slave, half-duplex protocol having data formatted into receive frames and transmit frames carried on separate wires. Both receive frame and transmit frame have the same structure and are synchronized with each other. Each frame comprises a flag byte followed by 8 device channels, each channel comprising 3 bytes space. The flag byte is used as a reference for frame synchronization. Physical connection is by way of two half duplex RS-485 multipoint links utilizing telephone cable for connection between devices.